Iron-based amorphous alloys are sought for their soft magnetic properties in the making of magnetic cores. They are manufactured by continuous rapid solidification of a stream of molten alloy cast on a moving chilled surface at speeds approaching 100 km per hour to output a very thin and ductile metal ribbon of various widths which can be cut at different lengths. Magnetic cores are then produced either by rolling a continuous ribbon or, by stacking multiple ribbon lengths. However, residual mechanical stresses are introduced into the alloy during casting, and applied stresses are added afterwards by bending or stacking the ribbon. These stresses will impair the magnetic properties and must therefore be removed from the ribbon when it adopts a final configuration into a core or, at least accommodated to a certain extent. Stress removal from the amorphous metal ribbon is generally accomplished by annealing the material in a furnace at an elevated temperature for a predetermined amount of time. Also, the magnetic properties are improved if a magnetic saturation field or a tensile strength is applied along the ribbon longitudinal axis during the furnace annealing treatment. Unfortunately, the furnace annealing treatment embrittles the alloy which becomes impossible to cut and difficult to manipulate. Embrittlement of iron-based amorphous alloys induced by furnace annealing has been a recurring problem for a long time.
A method for producing a distribution transformer kernel with a ferromagnetic amorphous metal ribbon is disclosed by Allan et al. in U.S. Pat. No. 5,566,443. A transformer kernel in the present document refers to the arrangement in the transformer comprising the electric coils, the core and the elements for supporting them together, without the transformer enclosure and surrounding accessories. In this patent, a number of electric coils are preformed, each having a portion with a shape of a sector of a circle. The preformed coils are then assembled together so that their portions combine to form a circular limb and, in order to construct the magnetic core, a continuous ferromagnetic amorphous metal ribbon is rolled up on a circular hollow mandrel located around the circular limb to produce a circular core. Before being rolled up, the amorphous metal ribbon has been previously furnace annealed under a magnetic saturation field on a second circular mandrel having the same external diameter as for the circular hollow mandrel, thus requiring a transfer of the annealed ribbon between mandrels.
Rolling-up-after-annealing of amorphous metal circular cores, although simple in appearance, remains a difficult task. The fact that the ribbon becomes brittle following the furnace annealing treatment makes it less convenient when it needs to be rolled up again on a second mandrel. Silgailis et al. in U.S. Pat. No. 4,668,309 have demonstrated in Table 2 of the patent that in each attempt to unroll and to roll up again a ferromagnetic amorphous metal ribbon of a furnace annealed circular core weighting around 50 kg at speeds up to 0.3 meter per second, the ribbon broke more than 60 times. Therefore, production of circular core made with rolling-up-after-annealing of an amorphous metal ribbon which has been previously furnace annealed in a roll is impractical due to the embrittlement of the amorphous alloy.
Shorter annealing times at higher annealing temperatures are believed to yield amorphous metal ribbons with greater ductility. However, there is a limit in trying to shorten the annealing time in a furnace due to a limit in heat transfer capacity within the core. Higher heat transfer capacity becomes possible by heat treating a single forwarded ribbon, under a tensile stress, in-line along a portion of its travelling path as disclosed in U.S. Pat. Nos. 4,482,402, 4288260, 5,069,428, and patent application US2008/0196795. Such apparatus are in-line ribbon annealing process. Once annealed, the ribbon is directly rolled-up on a reel mandrel or on a transformer kernel mandrel like the one disclosed in U.S. Pat. No. 5,566,443. Such apparatus would gain in productivity if means were provided at the input to maintain a continuous supply of ribbon and, at the output to ensure continuous production of rolls either on reel mandrels or on transformer kernel mandrels. According to paragraph [0080] in US patent application US2008/0196795, the output of the disclosed in-line annealing apparatus can comprise first and second winding spindles, so that it is possible, after winding a first core (or reel) over the first spindle, to cut the ribbon and to fit a head part of the ribbon onto the second spindle, in order to carry out the winding of a second core (or reel), without interrupting the manufacturing process. Paragraph [0084] further states that: it can be advantageous to use a magnetic spindle or a spindle with suction in order to immobilize the ribbon start on the spindle. However, the document does not teach nor show how to realize such continuous winding means and, does not include any means at the input for ensuring a continuous supply of ribbon.